Roasting of pyritous minerals.



- .PATENTED JAN. 1, 907.

I; I ous MINERALS.

JAN 28 1902.

No; 840,253- G. RAMBALDIN --ROASTING PYRIT,

Ar'rmoumn rum) GIOVANNI RAMBALDINI, OF MINIERA DI BOOCHEGGIANO, ITALY.

ROASTING OF PYRITOUSY MINERALS.

Specification of Letters Patent.

Patented. Jan. 1, 1907.

Application filed January 28, 1902. Serial No. 91,600.

To all whom it may concern:

Be it known that I, GIOVANNI RAMBALDINI, mining engineer, a subject ofthe King of Italy, residing at Miniera di Boccheggiano, Province ofGrosseto, in the Kingdom of Italy, have invented certain new and usefulImprovements Relating to the Roasting of Pyritous Minerals, of which thefollowing is a specification.

Desulfurizing roasting of pyritous minerals is preferably employed forpyritous minerals poor in copper, but may also be employed in the samemanner for mixed sulfid minerals by merely varying the temperature ofdesulfurization. The object is to convert metallic sulfids intosulfates-i. 6.,th6l1'lSOl1lble compounds of the metal into solublecompoundsand it is more particularly designed to serve as a substitutefor the roasting in a heap in the open air, it being based on the sameprinciple and utilized in the hydrometallic treatment of sulfid copperores which contain this metal in large proportions in the form of copperpyrites. In this caseas the only object of the treatment is nearlyalways the extraction of the copper and is therefore independent of themethod and the chemical formula employed for obtaining the copper as thefinal result, the contemporary presence of sulfate of copper and sulfateof iron derived from the sulfids is always prejudicialthe desulfurizingroasting in closed chambers, as hereinbefore stated, is effected in thefollowing manner by obtaining, first, the maximum yield of copper in asoluble form; second, the minimum yield of iron, also in a soluble form.The two characteristic reactions of this roasting process are, as far aspossible, as follows:

(I) F620U2S4+15O (2) ZFGSe-l- 110 I F620s+4SOa the first of which foreach atom gram of cop per develops three hundred and eleven greatcalories and the second for each atom-gram of iron two hundred and fortygreat calories.-

position is complete at 600, whereas the latter decomposes completelyonly above 700 750 centigrade.

Second. Crushing as small as possible, so as to increase the surface ofthe mineral exposed to the action of the oxidizing medium and also toavoid the well-known phenomenon of the formation of a core ofinsufl'lciently-desulfurized copper, which would be facilitated by theuse of large pieces.

' In order to. effect the first-mentioned condition, it is necessary toinsure a careful superintendence during the entire process and the exactand frequent determination of the temperature at the center of the masswhere the temperature is greatest and where it should not exceed 7 00centigrade. In order to effect the second condition in a practicalmanner and Without too much interfering with the circulation of the gasand the re newal of the oxidizing medium, it is necessary, first, tocarry out the process under shelter against the elements second, tocarry the crushing very far and classify the product with judgment inseparating the granulous portion from the powder; third, to roast thesetwo portions in the same mass by completely abandoning the horizontalStratification and, on the contrary, adopting the arrangement ofvertical columns or heaps.

The following is the practical procedure embodying the foregoingprinciples: After the preliminary breaking of the mineral the latter isground so as to form the largest possible quantity of granular material,after which the material 1s carefully screened into three separateheaps, vizfine, medium, and largefor recrushing. The fine portionrepreents as near as possible about the fifth part of the totalmaterial, and this is simply moistened with water, without, however, themaximum moisture which, it can imbibe. Under these conditions it can bekneaded roughly and can assume and retain the form of the vessel inwhich it may be sli htly compressed, but not so far as to enab e it tobe formed into blocks, as has been done heretofore. The mineral thusprepared is subj ected direct to roasting.

The roasting is carried out in closed chambers of rectangular form, and,in the accompanying drawings, Figure 1 represents a section, and Fig. 2a plan, of the same. Fig. 3is a perspective view of one of theopen-ended boxes.

The entrance of air takes place through a series of channels a a, formedin the base of the lateral walls and so arranged as to permit of beingcontinued in the interior in all directions by the insertion of drybricks, and thus forming a false floor which is permeable to air andleading to the free space left between the two floors. The egress of thegases is facilitated at the top through openings I) b.

The mineral is supplied by means of the small railway c, from which itis discharged into the funnels cl and then discharged, after roasting,through the lower doors a e, which during the roasting are kept closed.

The manner of operating is as follows: Upon the false floor are placedfagots s and pieces of wood, which during the first period serve as afilter for the air and facilitate the roasting of the lower strata toomuch exposed to cooling. The said strata are traversed by a certainnumber of small chimneys ff, constructed of dry bricks of convenientsize and designed to convey the air to the level of the flame.Alternately with these small chimneys there are placed upon the floor invertical piles square boxes g, of sheetiron, open at both ends andfurnished at their upper ends with strong handles. These boxes are notseen in Fig. 1, as the boxes are supposed to have been removed, theore-columns only being visible. After having constructed the smallchimneys ff to the height of the boxes the granulated mineral w ischarged through the funnels (Z,- and by taking proper precautions allthe free spaces are filled up to the same height. Then the finely-groundmoistened mineral is caused to fall into the boxes by pressing it welldown. After thus one strata has been prepared the next strata is formedin the same manner by lifting the boxes, without, however, completelywithdrawing the same and by continuing the small chimneys f f and againplacing a thin layer of fagots in the empty spaces between the boxes andthe said chimneys. Then the third and fourth strata, &c., are formeduntil a certain height h h is reachedi. a, the level for lighting. Uponreaching this point all the columns and small chimneys are interrupted,and the latter are covered with bricks and the whole leveled up withlarger-size minerals, and finally a good bed of sticks, wood, and coalis directly superposed, so as to form an arrangement interlaced withsmall channels containing wood and a little tow and petroleum. All thesaid channels are brought into intercommunication by providing twocrosschannels through the lateral walls within easy'reach by hand at theexterior in order to permit of the simultaneous lighting of allsuperposed combustible materials. Above this bed after havingdistributed a thin layer of granulated mineral the construction iscarried farther in the same manner as before,

but without the small chimneys, and is continued to the height of aboutone-third or one-fourth of the full height of charging. Lastly, theboxes (1 are removed and the materials g leveled and the whole coveredwith a thin layer of suitable mineral. In lighting the kiln it sufficesto set fire to the tow projecting to the exterior at the reach of thehand and at the same time start the draft. It is evident that changesmay be made in this process, especially with the object of eliminatingthe small chimneys f, which are cumbersome and expensive, and offacilitating the ingress of air into the interior of the columns ofmineral. After thus starting the kiln the fire should be vigilantly keptunder observation by so regulating the draft, as before mentioned, so asto prevent the temperature from exceeding 7 00 centigrade. Thepropagation of the fire usually takes place from top downward. It is,indeed, necessary that the fire should advance in the opposite directionto the movement of'the gases, and this as slowly as possible, so as tofacilitate and accelerate the oxidation of the upper strata. Thetemperature at the center, as before mentioned, must not exceed 700centigrade, so as not to cause the complete decomposition of the sulfateof copper, and it should be above 600 centigrade, so as to completelydecompose the sulfates of iron. However, owing to the losses of heat, itis but natural that this temperature is always decreasing from thecenter toward the exterior, where a portion of the sulfates of ironremains undecomposed. It will never be possible to eliminate by roastingonly all the soluble iron salt, and this residue of iron will remain ingreat preponderance in the ferric state because by means of the saidprocess, which is an energetic oxidizing process, the ferrous compounds(reduced and unstable) become converted little by little into ferriccompounds of a more stable nature. At first this would appear naturallyto constitute a nuisance but really the presence of the undecomposedferric sulfate must to a certain extent beconsidered a real necessityfor effecting the elimination of the copper. It is necessary'to maintainthe temperature below 700, and at about this temperature the sulfate ofcopper remains unchangedi. e., as CuSO,because, as is well known, thissalt when heated to dark red commences to decompose partially by formingdibasic sulfate, (Cu 80,, Cu 0.) However, this sulfate, although verysoluble in the presence of an acid, is only partly soluble in water, inthe presence of which, besides the normal sulfate SO Cu, it forms otherbasic sulfates always more complex and less s0luble, and thisnecessitates that during the attack on the roasted niineral'i. 6.,during lixiviationthere should exist or be formed within the water anagent which posseses the property of imparting to the basic sulfate ofcopper the acid radical required by it-for example, $0,. In this casethis agent may advantageously .be ferric sulfate, which, indeed, andespecially so at the moment when it commences to dissolve, produces anenergetic action ,upon the oxid of. copper and consequently upon thebasic sulfate of copper, (CuSOCuO,) in which the-combination between thebase CuO- and the salt may be kept veryweak. Thus 30.7 grand caloriesare developed for every atom-gram of sulfated copper and the change ofbases takes place, whereby hydrated eroxid of iron is precipitated. Thisis doub y useful in the present case, because if it permits, on the onepart, the dissolution of the copper it produces, on the other part, theprecipitation of a certain portion of the iron which still existed inthe solution. Parting from this principle the theory would naturally bethat the quantity of ferric sulfate still present in the mineral afterroasting ought to correspond to the base CuO, ready to be dissolvedafter the largest portion of the foregoing chemical eguation. However,in reality, if the amount 0 sulfur in the mineral is originally somewhatgreat the quantity of ferric sulfate may be increased and the excessafterward remains in the solutions. This, as has been said before, has anoxious effect on all the successive operations in thehydrometallurgical treatment, and it will be well to avoid the same asmuch as ossible by limiting from the beginning by tie roasting itselfthe excess of ferric sulfate to a minimum without, however, eliminatingit completely.

Therefore in order not to disturb theresiding' temperature in the massit is here proposed to have recourse to the action of steam upon themetallic sulfids, it being well known that these sulfids by reason ofthe heat and'the presence of steam form the respective oxids:

by developing sulfureted hydrogen. This reaction also takes place at lowtemperatures. For pyrites and copper pyrites it commences at 200centigrade, and if it takes place without the aid of water it is alwaysenothermic, and therefore most convenient for cooling the mass on fire.Further, this may be useful also for another reason, because the anagent it may be useful in-trying a practicalsolution for the fixing ofthesulfurous anhydrid.

Thus the present method of roasting in closed chambers, which by itselfnecessitates the collection of the products of combustion, will morethan any other be capable of supplying suitable means for the fixing ofthe 80,. For example, upon the base,of the foregoing considerations itwill be easy to calculate that a mineral which has only one percent. ofsulfur and ,three per cent. of copper when subjected to roasting inclosed chambers even if it yielded all its copper and also three. percent. of iron in form of sulfates may still produce gaswith 5.25 percent. of S0 and thus be suitable for application in any convenientmanufacture of sulfuric acid by the ordinary process in lead-chambers.This result with minerals so poor in sulfur would, on the contrary, beabsolutely impossible with any other process of roasting with intensiveworking.

The chimneys facilitate the access and the diffusion of the air'withinthe mass and render the ignition both safer and quicker. They arecovered with brick in order to prevent the ores from falling thereinwhile the same are charged and arranged in strata.

The purpose of the arrangement in vertical columns is that offacilitating the access of air and the circulation of gases throughoutthe same. A horizontal stratification of alternately finely-comminutedand granular material would, on the contrary, render the circulationextremely diflicult. It would then be necessary to limit the crushing inorder not to prevent the circulation of the gases entirely, while it isdesired, on the contrary, to crush the ores as finely as possible inorder to facilitate the oxidation, and consequently the obtaining ofsulfates.

No special arrangement is necessary to allow of a the fire advancingdownwardly. Should the combustion-gases advance upwardly-that is to say,the ignition take place near the bottomsaid gases after having passedthrough a certain mass of combustible material would become inert andunable to further entertain the combustion in the overlying strata. Ifthe mass were high, a part of the pyrites would not burn at all. If, onthe cbntrary, the ignition takes placeat the top, the stratum directlyunderlying the top stratum, which is burning, would be heated by theheat developed by said top stratum being contained in an oxidizingatmosphere, owing to the free oxygen in the air coming from below, andthe presence of the steam will also enter in combustion withoutdifiiculty. The above explanation as applied to the second stratum alsoapplies to a third stratum immediately underlying the second, and so onthe combustion proceeds downwardly, as in the case With 'a great numberof kilns and roasting apparatus, Without any difiiculty when thecombustion begins at the top and the air is introduced from below.

What I claim, and desire to secure by Letters Patent of the UnitedStates, is 1. The herein-described process for treating minerals whichconsists in effectually crushing the'materials for treatment, limitingthe maximum size between ten and fifteen millimeters, obtaining thegreatest pos sible quantity of granular material, roasting the same andseparating the granular portion. p

2. In the process of treating minerals, the step which consists inroasting the materials in a mass, and disposing the material in veritical columns and bringing all of said columns into intercommunication.

3. In a process for treating minerals, roasting the same in a mass,disposing the same in Vertical columns and bringing all of said columnsinto intercommunication, oxidizing the sulfids during roasting, andmaintaining the temperature at the center of the mass at substantially7009centigrade.

In testimony whereof I have hereunto set my hand, in presence of twosubscribing Witnesses, this 10th day of January, 1902.

GIOVANNI RAMBALDINI;

Witnesses EMILIO MASI, ANTONIO MORISANDI.

